record transaction sampling method and rate

Author: lobsterkatie

packages/core/src/request.ts

@@ -20,6 +20,10 @@ export function sessionToSentryRequest(session: Session, api: API): SentryReques
 
 /** Creates a SentryRequest from an event. */
 export function eventToSentryRequest(event: Event, api: API): SentryRequest {
+  // since JS has no Object.prototype.pop()
+  const { __sentry_samplingMethod: samplingMethod, __sentry_sampleRate: sampleRate, ...otherTags } = event.tags || {};
+  event.tags = otherTags;
+
   const useEnvelope = event.type === 'transaction';
 
   const req: SentryRequest = {
@@ -41,6 +45,11 @@ export function eventToSentryRequest(event: Event, api: API): SentryRequest {
     });
     const itemHeaders = JSON.stringify({
       type: event.type,
+
+      // TODO: Right now, sampleRate may or may not be defined (it won't be in the cases of inheritance and
+      // explicitly-set sampling decisions). Are we good with that?
+      sample_rates: [{ id: samplingMethod, rate: sampleRate }],
+
       // The content-type is assumed to be 'application/json' and not part of
       // the current spec for transaction items, so we don't bloat the request
       // body with it.

packages/tracing/src/hubextensions.ts

@@ -28,18 +28,6 @@ function traceHeaders(this: Hub): { [key: string]: string } {
   return {};
 }
 
-/**
- * Implements sampling inheritance and falls back to user-provided static rate if no parent decision is available.
- *
- * @param parentSampled: The parent transaction's sampling decision, if any.
- * @param givenRate: The rate to use if no parental decision is available.
- *
- * @returns The parent's sampling decision (if one exists), or the provided static rate
- */
-function _inheritOrUseGivenRate(parentSampled: boolean | undefined, givenRate: unknown): boolean | unknown {
-  return parentSampled !== undefined ? parentSampled : givenRate;
-}
-
 /**
  * Makes a sampling decision for the given transaction and stores it on the transaction.
  *
@@ -64,15 +52,33 @@ function sample<T extends Transaction>(hub: Hub, transaction: T, samplingContext
 
   // if the user has forced a sampling decision by passing a `sampled` value in their transaction context, go with that
   if (transaction.sampled !== undefined) {
+    transaction.tags = { ...transaction.tags, __sentry_samplingMethod: 'explicitly_set' };
     return transaction;
   }
 
   // we would have bailed already if neither `tracesSampler` nor `tracesSampleRate` were defined, so one of these should
   // work; prefer the hook if so
-  const sampleRate =
-    typeof options.tracesSampler === 'function'
-      ? options.tracesSampler(samplingContext)
-      : _inheritOrUseGivenRate(samplingContext.parentSampled, options.tracesSampleRate);
+  let sampleRate;
+  if (typeof options.tracesSampler === 'function') {
+    sampleRate = options.tracesSampler(samplingContext);
+    // cast the rate to a number first in case it's a boolean
+    transaction.tags = {
+      ...transaction.tags,
+      __sentry_samplingMethod: 'client_sampler',
+      __sentry_sampleRate: String(Number(sampleRate)),
+    };
+  } else if (samplingContext.parentSampled !== undefined) {
+    sampleRate = samplingContext.parentSampled;
+    transaction.tags = { ...transaction.tags, __sentry_samplingMethod: 'inheritance' };
+  } else {
+    sampleRate = options.tracesSampleRate;
+    // cast the rate to a number first in case it's a boolean
+    transaction.tags = {
+      ...transaction.tags,
+      __sentry_samplingMethod: 'client_rate',
+      __sentry_sampleRate: String(Number(sampleRate)),
+    };
+  }
 
   // Since this is coming from the user (or from a function provided by the user), who knows what we might get. (The
   // only valid values are booleans or numbers between 0 and 1.)
@@ -88,7 +94,7 @@ function sample<T extends Transaction>(hub: Hub, transaction: T, samplingContext
       `[Tracing] Discarding transaction because ${
         typeof options.tracesSampler === 'function'
           ? 'tracesSampler returned 0 or false'
-          : 'tracesSampleRate is set to 0'
+          : 'a negative sampling decision was inherited or tracesSampleRate is set to 0'
       }`,
     );
     transaction.sampled = false;